In some of the most famous experiments in embryology, Spemann and Mangold demonstrated the potential for brain formation is not restricted to specific cells in the ectoderm of early amphibian embryos, but is induced by cell contact with the underlying chordamesoderm. The central concept which emerged from their work, of cell interactions determining cell fate, remains a cornerstone of modern embryology. Nevertheless, over fifty years later, both the molecular signal that induces neural differentiation and the method of its communication remain to be discovered. We have begun to reinvestigate the problem of neural induction with modern microinjection techniques in embryonic ascidians, primitive marine chordates with a simple, archetypal development. We have studied several local species of ascidians with a view to obtaining the most amenable preparation for our studies. We have chosen Ascidia interrupta for its relatively largen and transparent eggs. These hermaphrodites produce fertile eggs and sperm throughout the year, and they self-fertilize with reasonable efficiency. Thus, control and experimental protocols can be carried out on a clutch of genetically identical embryos. Furthermore, the block to polyspermy does not reside in the extraembryonic membranes, so we have removed them enzymatically without disrupting development, which proceeds to the tadpole stage in less than 18 hours at room temperature. Microelectrodes will be used to inject specific pharmacological probes into single, identified blastomeres. The effect of these compounds on neuronal development will be determined by simultaneously filling the blastomeres in the presumptive neuroectoderm with fluorescent tracers of cell lineage (conjugated dextrans) and of cell coupling across gap junctions (Lucifer yellow). These experiments are designed to illuminate one of the most fundamental unsolved problems in biology: How does one cell alter the fate of its neighbor during vertebrate embryogenesis?